Dislocation prismatic

As an indenter creates an indentation it causes at least three types of finite deformation. It punches material downwards creating approximately circular prismatic dislocation loops. At the surface of the material it pushes material sideways. It causes shear on the planes of maximum shear stress under itself. Therefore, the overall pattern of deformation is very complex, and is reflected... 

The hardness of WC is associated with the fact that the array of W-atoms in the cores of glide dislocations changes from hexagonal prismatic to quasi-octahedral so the coordination number of the C-atoms changes from approximately six to approximately eight. This increases the local electron density so dislocation motion is resisted. 

Still another force will be present if a dislocation is curved. In such cases, the dislocation can reduce the energy of the system by moving to decrease its length. An effective force therefore tends to induce this type of motion. Consider, for example, the simple case of a circular prismatic dislocation loop of radius, R. The energy of such a loop is... 

Figure 11.13 Annealing prismatic dislocation loop taken as a circular array of vacancy...

Figure 11.15 Formation of prismatic dislocation by vacancy precipitation and collapse, (a) Excess vacancies dispersed in crystal, (b) Precipitation of excess vacancies, (c) Collapse of vacancy precipitate to form dislocation loop.

Solution. The helical dislocation may be regarded as equivalent to a stack of circular prismatic edge dislocation loops of radius a as illustrated in Fig, 11.16. This may be confirmed by realizing that the stack of loops can be converted into a helix in a conservative fashion by cutting each loop at its intersection with AB and then sliding... 

Figure 11.16 (a) A stack of four prismatic edge dislocation loops perpendicular to AB... 

Figure 23.4 Prismatic dislocation punching at spherical precipitate, (a) A dislocation dipole loop is generated in the interface. One side expands into the matrix while the other remains in the interface, (b) Segments of the loop in the matrix glide downward to form additional loop length in the interface, (c) The loop in (b), which is partially in the interface and partially in the matrix, pinches together at its lowest point and splits into two loops, with one remaining in the interface and the other gliding into the matrix. From Porter and Easterling [4],...

Most wurtzite GaN films have been grown on either 6H-SiC(0001) (see Datareview A7.8) or sapphire (A1203) substrates. The orientation of sapphire most frequently used is C-plane (0001) although there have been some structural characterisation studies made for growth on A-plane (1120) [1-4] and R-plane (0112) [1,2,5-7] substrates. Other defects found in the a-phase include inversion domain boundaries, prismatic faults, nanopipes, pits, voids and cracks. The limited structural information available on bulk single crystals of a-GaN shows that they contain a low density of line dislocations and stacking faults near inclusions [12] (see Datareview A7.5). 

Figure 5.5. Schematic diagrams showing (a) a random distribution of vacancies, (b) condensation of a cluster onto a single plane, and (c) the collapse of the planes to form a prismatic dislocation loop.

Figure 5.IS. Schematic diagram showing (a) a prismatic dislocation loop lying in the plane of the foil and (b) its image when g b = 0.

Ashby and Brown (1963b) extended the analysis to cover the type of strain field found around platelike precipitates whose mismatch with the surrounding crystal is appreciable only in a direction normal to the plate. The analysis is also applicable to the strain normal to the plane of a prismatic dislocation loop of Burgers vector b. When the plane of the loop or precipitate is more or less normal to the foil, the images are similar to that... 

Figure 5.24. Variation of the 20-percent image width with g, b, and 0 for a prismatic dislocation loop (or a platelike inclusion) in an isotropic crystal matrix for several values of / ,. Thickness of foil = s = 0 r" = 10/g,...

Figure 9.8. BF image (g = lOTl) showing prismatic dislocation loops (b = <1120 and strain-free bubbles in wet synthetic quartz after heating at 550°C for 2 hours. (From McLaren et al. 1989.)...

Small prismatic loops viewed edge-on may give rise to a characteristic contrast of two symmetrical lobes, particularly when g is normal to the plane of the loop, as discussed in Section S.7.2. The contrast and orientation of the (arrowed) dislocation loop viewed edge-on in Figure 9.11 (a, b) are consistent with its being a prismatic loop of b = [0001]. 

Thus, the main loop (A, D) is a prismatic edge-dislocation loop (the Burgers vector is normal to the plane of the loop) and can expand in its plane only by climb. Segments of the loop have dissociated into pairs of partial dislocations (B, C and E, F), presumably by the reaction... 

Doukhan and Doukhan (1986) suggested that the climb dissociation is due to the precipitation of point defects on the prismatic loops when the specimens are cooled. The equilibrium concentration and the mobility of the point defects are both expected to be very high at 1,400°C, thus favoring deformation by dislocation climb. 

Begining of the simulation (left), after deformation (right). Basal dislocations glide horizontally (horizontal lines), dislocations in the prismatic plane vertically (white vertical lines). The center of the cylinder is represented. 

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